New bookmarklet:
!grid (drag to Links-bar)
Click once to overlay the current page with a div containing a 50x50 grid pattern. Click again to hide. Should be handy for testing alignment, quick dimension estimates, etc. Tested in IE6 and Firebird 0.7.
(Suggestions for better grid patterns are welcome; grid9.gif looks OK here but looks a bit iffy on lighter backgrounds.)
Update: here's a new version using a different grid pattern, having alternating bright and dark pixels per P01's suggestion, and without the inner 10x10 lines:
!grid2
 { void(dov=document.createElement('div')); with (dov.style){void(position='absolute',top='0px',left='0px',width='100%',height='3000px',MozOpacity=.5,filter='Alpha(opacity=50)',backgroundImage='url(http://milov.nl/g/grid9.gif)');} void(document.body.appendChild(dov)); } void(dov.style.display = dov.style.display == 'block' ? 'none' : 'block');){kind=link}
 { void(dov=document.createElement('div')); with (dov.style){void(position='absolute',top='0px',left='0px',width='100%',height='3000px',MozOpacity=.8,filter='Alpha(opacity=80)',backgroundImage='url(http://milov.nl/g/grid11.gif)');} void(document.body.appendChild(dov)); } void(dov.style.display = dov.style.display == 'block' ? 'none' : 'block');){kind=link}
milov: To enhance the visibily, you could alternate bright and dark pixels on the lines of the grid pattern.
JQN: wow, using a paper tape is so roots. I put some stickytape on the side the monitor of my Atari to see how much cpu time I had to remove to fill a full Vertical BLank. ^__^